Fuel Guage Goes Up Then Partly Fades -- 72 Swinger

Does the temp gauge work? I would FOR CERTAIN perform a resistor test---sub a resistor for both the temp and fuel senders and see what the gauges do. The IVR supplies both. One thing you might actually try, is to remove the (temporarily) sender wire at the left kick panel connector where the tail harness comes up. Now "rig" and extend the wire through into the engine bay (you can knock the clutch rod plug/ grommet out temporarily, run a wire and hook it to the temp gauge

FOR SURE determine if the temp gauge is "doing the same."

If not, might be loose stud nuts on the gauge, or the gauge itself, or poor connection in the path, or poor connection at the sender wiper arm (where it contacts the resistor)